Electrolytic etching of aluminum foil



Great liritain No Drawing. Fiied Feb. 11, 1960, Sor. No. 7,995 Qlairns priority, application Great Britain Feb. 20, 1959 4 Claims. (Cl. 204141) This invention relates to improvements in the treatment of aluminium (which term is intended to include aluminium-base alloys) and is particularly concerned with the treatment of aluminium to increase the effective surface area thereof. Aluminium and, in particular, aluminium foil so treated is of particular value in the construction of electrolytic capacitors which include an electrode of aluminium on the surface of which is an electrically formed (ii-electric film of oxide, the capacitance of such capacitor depending upon the area of filmed electrode surface in contact with the electrolyte between it and the complementary electrode.

It has been proposed to increase the effective surface area of aluminium and, in particular, aluminium foil by both a chemical etching process and by an anodic electrolytic etching process in which a smooth direct current is employed or in which a uni-directional uninterrupted current of fluctuating magnitude is employed. The increase in the effective surface area of aluminium so treated is largely due to the production of very fine and relatively deep pores and very little is due to the readily apparent coarse etching of the surface which occurs although the latter is responsible for the larger proportion of the metal lost during the treatment. Coarse etching also leads to mechanical weakening of the metal and is particularly undesirable in aluminium foil as there is a [tendency for the foil to become perforated. The known etching processes referred to above produce an undesirably high amount of coarse etching on aluminium so treated and an undesirably high loss of the metal treated with the result that the increase of the effective surface area of the aluminium is usually limited to values appreciably lower than the ultimate attainable.

It is an object of the present invention to provide an improved process for treating aluminium to increase the effective surface area thereof which process shall not produce coarse etching of the surface nor loss of the metal being treated to the same extent as known processes;

According to the present invention a process for treating aluminium or an aluminium base alloy to increase the effective surface area thereof comprises auodically electrolytically etching the metal using interrupted pulses of uni-directional current.

The invention also extends to aluminium or aluminium base alloy foil treated by the process of the immediately preceding paragraph and to capacitors embodying such foil.

We have [found that with the process according to the invention only a very small amount of coarse etching occurs and it is possible to increase the effective surface area of 0.004 inch thick aluminium foil by a factor of up to and more for the removal of about 10% by weight of the metal. This is in direct contrast to known processes which remove from 4 to 6 times this amount of metal rates atent K 3,085,950: Patented Apr; 16, 1963 ice for an increase in the effective surface area of the same order.

The electrolyte employed must conform to the following conditions:

(1) The chemical attack which it exerts on the aluminium being treated must be low so that at least by Weight and preferably more of the metal removed during treatment is due to the electric current.

(2) It must have a relatively high electrical conductivity.

(3) It should not be unduly modified by the current flowing therethrough.

In addition to fulfilling the above conditions it is preferred that the electrolyte should have a pH value of from 5 to 8.

Aqueous solutions of the chlorides, bromides, iodides and nitrates of sodium, potassium, magnesium and ammonium have been found suitable. Thus the process may be operated with electrolytes other than the chlorides and this is particularly advantageous when it is desired to obtain a large increase in the effective surface area of the aluminium without danger of contamination by chlorides which are generally difii-cult to remove completely from the etched surface and traces of which are highly deleterious during subsequent stages of capacitor manufacture.

A good uniformly etched surface with a high gain of surface area may be obtained by the process according to the invention over a wide range of electrolyte concentrations. For example, aqueous solutions of sodium chloride have been used in concentrations of from 5 .grms/ litre to saturation, but solutions containing from -150 grms./ litre are preferred in order to obtain a reasonably high conductivity. In general, the concentration of electrolyte used is preferably in the range of 100-200 grms./ litre. More concentrated solutions confer no extra advantages whilst introducing other problems of crystallisation, creep and extra precautions in washing as well as extra expense.

The pulse duration is desirably from 5 to 100 useconds and is preferably of the order of 20 useconds and the current falls to zero during the interval between pulses.

The pulse repetition frequency may be selected from the range of 20 c./s. to 6 kc./s. but it has been found that the electrical efiiciency of the process falls, as does also the maximum surface increase, as the frequency is raised. If the maximum surface increase is required the operating frequency is preferably about 100 c.p.s., although higher frequencies may be employed for more moderate surface gains.

The mean current density of the pulses is important as it should not be sufficiently high to cause fusion at the surface of the aluminium being treated. It is preferred that the mean current density over each current pulse should be from 3 to 50 amps/sqcm. and preferably within the range of 10 to 20 amps./sq.cm., pulses of longer duration requiring the lower values of mean current density during the pulse, and vice versa, so that the total energy delivered during the pulse is not high enough to cause fusion at the surface and destroy the character of the etch. It is preferred that the current pulses should be approximately rectangular.

The temperature of operation of the process has not been found to be very critical, although the efficiency of the process was found to fall off at temperatures above 3 40 C. It is therefore preferred that the temperature of the electrolyte should not exceed 40 C.

Some examples of the invention will now be described:

Example 1 An aluminium foil of 99.99% purity 0.004 inch thick was cleaned for 1 minute in 4% caustic soda solution at 60 C. Then it was etched anodically for minutes using aluminium cathodes in an aqueous solution of 150 grms./litre of sodium chloride at C. containing 0.5% borax and sufficient hydrochloric acid to bring the solution to a pH of approximately 7. The current consisted of nearly square pulses at 15 volts and 20 mi croseconds duration with a repetition frequency of 66 c./s. These current pulses produced a mean current density over each pulse of about 10 amps. per sq. cm. The foil lost 8% of its weight and after forming at 300 volts in a solution of 2% boric acid and 0.1% borax the capacity was 10 times that of an unetched foil similarly formed.

Example 2 An aluminium foil of 99.99% purity 0.004 inch thick was treated similarly to Example 1 but with a treatment time at 10 minutes and a repetition frequency of 140 c./s. The capacity increase was 10 times and the foil lost 12% of its weight.

Example 3 An aluminium foil at 99.99% purity 0.004 inch thick was treated similarly to Example 2, but with a treatment time of 2 minutes. The capacity increase was 4 times and the weight loss approximately 2%.

Example 4 An aluminium foil of 99.99% purity and 0.004 inch thick was anodically etched for 5 minutes in an aqueous solution containing 150 grms./litre of magnesium chloride at 20 C. using electrical pulses of 30 microseconds duration at 22 volts and a repetition frequency of 140 c./s. The capacity increase at 300 volts was 4.5 times, and the foil lost about 3% of its weight.

Example 5 An aluminium foil of 99.99% purity and 0.004 inch thick was etched for 5 minutes in an aqueous solution containing 200 grms./litre of potassium nitrate at 20 C., using positive pulses of 20 microseconds duration at 21 volts and a repetition frequency of 140 c./s. The capacity increase was 6.0 times, determined at 300 volts.

Example 6 An aluminium foil of 99.99% purity and 0.004 inch thick was etched in 150 grms./litre sodium chloride solution at 75 C. for two minutes. The current consisted of nearly square pulses at 19.5 volts and 20 microseconds duration with a repetition frequency of 140 c./s. The capacity increase was 2 times, measured at 300 volts.

It can be seen by comparison with Example 3 that at the temperature of operation employed the process was less efiicient.

Example 7 Using the same electrolyte and anode material as in Example 7, but with a repetition frequency of 1500 per second, a pulse duration of 10 microseconds and mean 5 current density during each pulse of 20 amps/sq. cm. the increase in surface area, determined in the same manner, was 5 times.

Example 9 An aluminium foil of 99.99% purity and 0.005 inch thickness was etched in an aqueous solution of 150 grms/ litre sodium nitrate for 15 minutes, the current pulses being supplied by successive discharges 20 times a second from a condenser bank charged up to 420 volts. The foil lost 4.8% Of its Weight in etching and the surface area was increased 4.1 times over that of unetched foil as determined by measuring the capacity when formed to 460 volts in a 2% boric acid/0.1 sodium borate solution. It will be appreciated that ammonium borate may be substituted for sodium borate.

It should be noted that while Examples 7 and 8 illustrate the operation of the process at higher repetition frequencies the other conditions are not necessarily optimum for these frequencies.

In the above examples the etched foil was cleaned and formed in such manner as to avoid excessive hydration. In particular, it was Washed in cold water after etching and before forming. The weight of aluminium removed from the anodic aluminium during etching is in most cases considerably less than might be anticipated from the current actually passing through the cell and it will be appreciated therefore that removal of aluminium is not the only electrode process taking place. It is possible to operate the process under conditions in which the anode efliciency of removal of aluminum approached (as in the case of Example 9) but such conditions are not often the best for securing large increases in surface area in a short time and with the minimum loss of metal.

It will be appreciated that the process of the present invention offers the advantage that a relatively large increase in the surface area of the aluminium treated may be achieved for very small metal losses and that a substantially uniform etching is obtained with a relatively low amount of coarse etching so that the danger of metal perforation is correspondingly reduced as is the loss of mechanical strength. It follows that it is possible to treat thinner foils than has been possible hitherto with the subsequent saving in weight and reduction in cost. Good results have been obtained on any temper or purity of aluminium, and the highest purity aluminium can be etched substantially uniformly by the process of the present invention. It follows that the metal to be treated by the process of the present invention does not require special production techniques or long process anneals.

What we claim is:

1. A process for treating aluminium and aluminium base alloys to increase the effective surface thereof, comprising anodically electrolytically etching the metal for from 2 minutes to 15 minutes using interrupted pulses of uni-directional current, wherein the pulse duration is from 5 to 100 microseconds, the pulse repetition frequency is in the range of 20 c.p.s. to 6 k c.p.s., and wherein the mean current density over each current pulse is from 3 to 50 amps./sq.cm., and wherein the electrolyte comprises an aqueous solution of one of the compounds selected from the group consisting of the chlorides, bromides, iodides and nitrates of sodium, potassium, magnesium and ammonium, and the electrolyte contains not less than 5 grms./litre of one of the compounds in the group specified, and the temperature of the electrolyte is not greater than 40 C.

2. A process for treating aluminium and aluminium base alloys in accordance with claim 1, wherein the current falls to zero during the interval between pulses.

3. A process for treating aluminium and aluminium base alloys to increase the effective surface thereof, comprising anodically electrolytically etching the metal for from 2 minutes to 15 minutes using interrupted pulses of uni-directional current, wherein the pulse duration is from 5 to 100 microseconds, the pulse repetition frequency is 4. A process for treating aluminium and aluminium in the W118i? 0f 20 -P- 6 4 and wherein the base alloys in accordance With claim 3, wherein the curmean current density olfif each Current Pulse is from 3 rent falls to Zero during the interval between pulses.

to 50 amps/sq. cm., and wherein the electrolyte comprises an aqueous solution of one of the compounds se- 5 References C5335 in the fil f this patent lected from the rou consistin of the chlorides, bromides, iodides aii d ni irates of sidium, potassium, mag- UNITED STATES PATENTS nesium and ammonium, and the electrolyte contains 100- 2,75 5,237 Turner -Fuly 17, 1956 200 guns/litre of one of the compounds of the group 2,755,238 Turner July 17, 1956 specified, and the temperature of the electrolyte is not 10 2,930 741 Burger et 1 M 129, 1960 greater than 40 C. 

1. A PROCESS FOR TREATING ALUMINIUM AND ALUMINIUM BASE ALLOYS TO INCREASE THE EFFECTIVE SURFACE THEREOF, COMPRISING ANODICALLY ELECTROYLTICALLY ETCHING THE METAL FOR FROM 2 MINUTES TO 15 MINUTES USING INTERRUPTED PULSES OF UNI-DIRECTIONAL CURRENT, WHEREIN THE PULSE DURATION IS FROM 5 TO 100 MICROSECONDS, THE PULSE REPETITION FREQUENCY IS IN THE RANGE OF 20 C.P.S. TO 6 K C.P.S., AND WHEREIN THE MEAN CURRENT DENSITY OVER EACH CURRENT PULSE IS FROM 3 TO 50 AMPS./SQ.CM., AND WHEREIN THE ELECTROLYTE COMPRISES AN AQUEOUS SOLUTION OF ONE OF THE COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF THE CHLORIDES, BROMIDES, IODIDES AND NITRATES OF SODIUM, POTASSIUM, MAGNSIUM AND AMMONIUM, AND THE ELECTROLYTE CONTAINS NOT LESS THAN 5 GRMS./LITRE OF ONE OF THE COMPOUNDS IN THE GROUP SPECIFIED, AND THE TEMPERATURE OF THE ELECTROLYTE IS NOT GREATER THAN 40*C. 